The iOS operating system for mobile devices is created and developed by Apple Inc. and distributed exclusively for Apple hardware.
A mobile device with the operating system typically has a so-called locked state where the screen of the device is off and some of the applications may be executing in the background. However, in this locked state with the screen off, the user would first need to wake up the device by pressing a so-called “home” button which would switch on the screen and allow the user to then unlock the device to obtain access to the full functionality of the mobile device and access the relevant applications or features.
In the locked state, pressing a power on/off button will also switch on the screen. Holding the power on/off button down for an extended period of time will switch off the device. Pressing the power on/off button twice in quick succession will have no further effect.